Methoxyphenylethynyl, methoxypyridylethynyl and phenylethynyl derivatives of pyridine: Synthesis, radiolabeling and evaluation of new PET ligands for metabotropic glutamate subtype 5 receptors

We have synthesized three different PET ligands to investigate the physiological function of metabotropic glutamate subtype 5 receptors (mGluR5) in vivo: 2-[¹¹C]methyl-6-(2-phenylethynyl)pyridine ([ ¹¹C]MPEP), 2-(2-(3-[¹¹C]methoxyphenyl)ethynyl)pyridine ([¹¹C]M-MPEP) and 2-(2-(5-[¹¹C]methoxypyridin-3-yl) ethynyl)pyridine ([¹¹C]M-PEPy). [¹¹C]Methyl iodide was used to label the compounds under basic conditions, and a Pd(0) catalyst was applied to label [¹¹C]MPEP in a Stille coupling reaction. In vivo microPET imaging studies of the functional accumulation of radiolabeled ligands were conducted in 35 rats (Sprague-Dawley, 8 weeks old male, weight of 300 g). Specific binding was tested using pre-administration of unlabeled mGluR5 antagonist 2-methyl-6-(2-phenylethynyl)pyridine (MPEP) (10 mg/kg iv 5 min before radioactivity injection). In the radiolabeling of [¹¹C]MPEP, [ ¹¹C]M-MPEP and [¹¹C]M-PEPy, a specific radioactivity of 700-1200 mCi/μmol and over 97% radiochemical purity were obtained. The microPET studies showed these three radiolabeled mGluR5 antagonists having the highest binding in the olfactory bulb followed by striatum, hippocampus and cortex. Pre-administration of the mGluR5 antagonist MPEP induced a 45.1% decrease in [¹¹C]MPEP binding, a 59.7% decrease in [ ¹¹C]M-MPEP binding and an 84.6% decrease in [¹¹C]M-PEPy binding in the olfactory bulb at 5 min. The feasibility of synthesizing high-affinity and high-selectivity ligands for mGluR5 receptors and their suitability as PET imaging ligands for mGluR5 receptors in vivo are demonstrated.